example/calc: add help message

examples/README.md

@@ -0,0 +1,15 @@
+# Examples
+
+## Calculator
+
+Fun calculator based on environment variables
+
+```
+E=2*2-1-1+5*3-10 cargo run --example calc --all-features
+```
+
+Limits:
+
+- Supports `*`, `+`, `-`
+- You cannot start from a negative number. `E=-10`. Solution: start from `0`.
+  `E=0-10`.

examples/calc.rs

@@ -1,13 +1,34 @@
 use enve::core::SepVec;
 
+type MinusVec<T> = SepVec<T, '-'>;
 type PlusVec<T> = SepVec<T, '+'>;
 type MulVec<T> = SepVec<T, '*'>;
 
+const HELP_MESSAGE: &str = "
+USAGE:
+E=10+10*2+4 cargo run --example calc --all-features
+";
+
 fn main() -> Result<(), enve::Error> {
-    let res: f32 = enve::get::<PlusVec<MulVec<f32>>>("E")?
+    let res: f32 = enve::get::<PlusVec<MinusVec<MulVec<f32>>>>("E")
+        .map_err(|err| {
+            match err {
+                enve::Error::NotPresent => eprintln!("The expression was not found"),
+                rest => eprintln!("ERROR: {}", rest),
+            }
+
+            eprintln!("{}", HELP_MESSAGE);
+            std::process::exit(0);
+        })
+        .unwrap()
         .iter()
-        .map(|m| m.iter().product::<f32>())
-        .sum();
+        .map(|p| {
+            p.iter()
+                .map(|m| m.iter().product::<f32>())
+                .reduce(|acc, v| acc - v)
+                .unwrap_or_default()
+        })
+        .sum::<f32>();
 
     println!("result: {}", res);